This invention relates generally to the detection of neutron flux in a nuclear power plant, and more particularly to an improved flux thimble.
A typical pressurized water reactor nuclear power plant includes a reactor vessel which contains nuclear fuel, a coolant (water) which is heated by the nuclear fuel, and means for monitoring and controlling the nuclear reaction. The reactor vessel is typically of a cylindrical configuration, provided with a hemispherical bottom and a hemispherical top which is removable. Hot water is conveyed from and returned to the vessel by a reactor coolant system which includes one or more reactor coolant loops, usually three or four loops, depending upon the power-generating capacity of the reactor. Each loop includes a pipeline to convey hot water from the reactor vessel to a steam generator, a pipeline to convey the water from the steam generator back to the reactor vessel, and a pump. The steam generator is essentially a heat exchanger which transfers heat from the reactor coolant system to water from a source that is isolated from the reactor coolant system, with the resulting steam being conveyed to a turbine to generate electricity. During operation of the reactor, the water within the vessel and the coolant system is maintained at a high pressure to keep it from boiling as it is heated by the nuclear fuel.
Nuclear fuel is supplied to the reactor in the form of a number of fuel assemblies. Each fuel assembly conventionally includes a base element called a bottom nozzle and a bundle of fuel rods and tubular guides which are supported on the bottom nozzle. The fuel rods have cylindrical housings which are filled with pellets of fissionable material enriched with an isotope of uranium, typically U235. The tubular guides accommodate measuring instruments and movably mounted control rods of neutron-moderating material. A typical fuel assembly for a pressurized water reactor is about 4.1 meters long, about 19.7 centimeters wide, and has a mass of about 585 kilograms, 196 such fuel assemblies being supported parallel to one another on a core plate within the reactor vessel of a typical four loop reactor. After a service life during which the U-235 enrichment of the fuel assemblies is depleted, the reactor is shut down, the pressure within the vessel is relieved, the hemispherical upper cap of the vessel is removed, and the old fuel assemblies are replaced by new ones.
A number of measuring instruments are employed to promote safety and to permit proper control of the nuclear reaction. Among other measurements, a neutron flux map is generated periodically, such as every 28 days, using data gathered by neutron flux detectors which are moved through a number of selected fuel assemblies located across the core. In order to guide the flux detectors within the fuel assemblies, closed stainless steel tubes known as flux thimbles extend through the bottom of the reactor vessel and into the fuel assemblies which have been selected as measuring sites. While the details of the detectors and their respective drive units are not illustrated herein, the operation of the detectors and the operation and processing of information are described and illustrated in U.S. Pat. No. 3,858,191, while details of the methods employed to monitor neutron flux in a nuclear reactor are described in U.S. Pat. Nos. 3,932,211, and 4,255,234, each of which is assigned to the assignee of the present invention, and is incorporated herein by reference.
Conventional flux thimbles have several shortcomings. A considerable amount of turbulence exists during operation of a reactor in regions to which the flux thimbles are exposed. Such turbulence vibrates the flux thimbles and causes wear to an undesirable extent. Simply increasing the size of the flux thimble would reduce the vibrations caused by such turbulence, but would also further complicate matters. The inside diameter of conventional flux thimbles must be manufactured to a very high standard of quality for surface finish, much higher than commercial grade tubing, since the interior portion of the flux thimble must permit rapid and accurate placement of the detector therein. Therefore, if the inside diameter of a flux thimble is maintained to promote movement of the detectors therein while the outside diameter is increased in order to reduce vibrations caused by turbulence within the reactor, the thickness of the flux thimble wall is increased thereby making the flux thimble stiffer and more likely to cause difficulty with insertion and retraction thereof into the reactor, operations that must take place at each refueling. In addition to the increase in stiffness caused by a flux thimble having a large outside diameter, the reduced annular gap between the larger flux thimble and its associated guide tubing will cause an increase in the number of contact surfaces therebetween resulting in higher friction, and again increased difficulty in the insertion and retraction of the flux thimble before and after refueling. Likewise, if both the outside and inside diameters of a flux thimble are increased to reduce vibration caused by turbulence in the reactor as well as to prevent an increase in stiffness, the increased inside diameter will provide additional clearance between the interior walls of the flux thimble and its associated detector which may result in problems of flux detector drive cable buckling or kinking as the detector is pushed into the thimble. It would, therefore, be desirable to provide a flux thimble which minimizes the potential for vibrations caused by turbulence in the reactor, while at the same time facilitates the insertion and retraction of the flux thimble and detector contained therein.
Another aspect which must be considered once an acceptably sized flux thimble is determined is the problems associated with the manufacture of such a flux thimble. Because of their critical nature, flux thimbles must be drawn in a continuous length from a billet of steel, the length of conventional flux thimbles often exceeding 120 feet. While the outside diameter need not be manufactured to the same high quality standards for surface finish as the inside diameter of conventional flux thimbles, the effort to develop a new flux thimble size becomes quite expensive and requires long lead times since retooling is required. It would, therefore, be desirable to adapt conventional flux thimbles by increasing their size only in the vicinity to which they may be exposed to vibration causing turbulence.